1. Field of the Invention
The present invention relates to the heat treatment of materials in an artificial atmosphere. More specifically, the present invention relates to the heat treatment of metals and alloys in an atmosphere substantially purged of oxygen through the use of a bi-phasic cryogen.
2. Description of the Related Technology
The production of finished metal products is carried out through a series of heat treating processes. Extracted raw metal ores are generally heated in furnaces in which ore reduction and smelting take place. Heating the materials into molten form allows the metal to be separated from impurities and allows the molten metal to be uniformly blended with other materials and metal to form alloys and metals of different grades. Once a desired composition is achieved the molten metal is removed from the furnace and allowed to cool in the form of ingots or slabs.
The ingots and slabs are then processed into the desired product form and shape, i.e., bar, sheet, strip, tube, wire. The typical forming and shaping process is generally carried out in a rolling mill furnace. In a rolling mill, ingots and slabs are heated so as to become more malleable and thereby more easily shaped into the desired product form. The heated ingots and slabs are then rolled, i.e., they are passed between opposed rolls in the cavity of the mill whereby they undergo an increase in length and a reduction in height or depth. Generally, it is not possible to reduce large slabs of metal into desired product form by a single pass through a pair of rolls. The forming process usually requires passing the metal several times through the same pair of rolls, wherein the rolls are progressively brought into abutment and the product is brought into its final shape. Alternatively, metals can be passed through a rolling train, wherein a series of rolls with gaps of diminishing width are provided in a successive relationship that conclude with the product being pressed into its final product shape.
Other forming and shaping processes in the art that generally require the heat treating of materials in furnaces include, but are not limited to, sintering powders, brazing metals and sealing glass to metals. As understood by one of ordinary skill in the art, an oxide layer (i.e. mill scale) is formed on the surface of oxidizable materials, particularly metals and alloys, whenever such a material is heat-treated in the presence of oxygen. This oxide layer must be removed, or preferably prevented from forming, before any successive forming or subsequent processing steps can be performed.
Accordingly, there has been a long-felt, yet unresolved, need in the art of metal fabrication to provide a method and apparatus for heat treating metals and alloys that reduces or prevents the formation of an oxide layer on the treated material's surface. This need is particularly acute in the annealing process, especially in the annealing of exotic metals and alloys. By "exotic," it is meant those comparatively rare specialty metals and alloys that may be particularly susceptible to oxidation, or otherwise have a high affinity for oxygen. Representative exotic metals include, but are by no means limited to, zirconium, titanium, molybdenum, tantalum and columbium.
Annealing is the process through which stresses and distortions in formed metal products are removed. Annealing generally involves the heating of a product to an effective temperature for a period long enough to allow the molecular structure of the material to adjust to a more uniform arrangement, and then controlling the cooling of the material such that the uniform arrangement can be maintained in the final product. Annealing is an important step in the finishing process of metal products. It is through annealing that a uniform and strong product being substantially free of weak spots and distortions is ensured.
Annealing of metal products generally involves several heating and cooling cycles to ensure uniformity of the finished product. As will be appreciated by one of ordinary skill in the art, each such cycle involves passing the metal product through the chamber of a furnace. The presence of oxygen in the furnace results in the formation of an oxide layer on the product's surface with each pass through the furnace. This layer must be removed from the product before the product can be sent through the furnace for the next heating and cooling cycle.
Removal of the oxide layer generally involves submerging the metal product in an acid bath to remove the oxide layer by corrosion. This "pickling" process necessitates the use of large volumes of acids, such as sulfuric acid, nitric acid and hydrofluoric acid. The presence and use of these acids on-site poses significant health, safety and environmental concerns. The acids must be shipped, delivered, stored and used in large quantities. In addition, pollution control and disposal of these acids is also of great concern and a considerable operating expense. Accordingly, there has been a long-felt need in the art to devise a method and apparatus that allows for the reduction or elimination of the need to pickle products during annealing and finishing processes. A similar need exists in other heat treating processes that ultimately result in the need to pickle products before successive or subsequent processing and finishing operations can be undertaken.
Prior art methods have failed to satisfy these long-felt needs. One such method prescribes the use of a completely fluid tight furnace chamber. The furnace chamber is then vacuum evacuated of substantially all ambient oxygen prior to heating the material to be treated. This process requires a special vacuum furnace and is generally only suitable for small batch processes. Further, the furnace must be capable of preventing the leaching of outside ambient air into the process in order to prevent a corrupting of the entire process. The use of a vacuum furnace also results in the need for a substantially long cooling period which lowers plant productivity. In addition, a vacuum process can be prohibitively expensive for many metals. Estimates on the price of operating a vacuum furnace range from $400-$600 per hour. Thus, there remains a need in the art for a less expensive, non-vacuum process that is suitable for large volume, continuous annealing and heat-treating processes.
Another common prior art method involves the purging of ambient oxygen from the furnace chamber by the introduction of an inert gas blanket. This method requires a continuous flow of gas to provide enough gas pressure in the chamber to prevent the ambient, oxygen rich air from reentering the chamber area. Even with a substantially fluid tight chamber, this process requires an extraordinarily large volume of gas to be used during the process and yet still fails to keep the concentration of residual oxygen low enough to prevent the formation of an oxide layer on most metal products. This is particularly true with respect to the easily oxidizable specialty metals, which still must undergo acid pickling despite the use of inert gases. Thus, there still remains a need in the art to achieve low residual oxygen concentrations through a purging process without having to use substantial volumes of inert gases or reach excessive pressures.